In automation technology, field devices are used for registering and/or influencing process variables. Examples of such field devices are fill level measuring devices, mass flow rate measuring devices, pressure and temperature measuring devices, etc., which, as sensors, register the corresponding process variables fill level, flow rate, pressure and temperature, respectively.
For influencing process variables, the field devices may include actuators, which, e.g. as valves, can change the flow rate of a liquid in a section of pipeline or, as pumps, the fill level in a container.
In principle, all devices installed near a process and which deliver, or process, process-relevant information, are referred to as field devices.
Field devices are available from the firm Endress+Hauser in many different forms.
Normally, field devices as used in modern industrial plants are connected via bus systems (PROFIBUS, FOUNDATION FIELDBUS, etc.) to superordinated, or higher level, units (control system or units). These superordinated units serve for, among other things, process control, visualization, and monitoring, as well as for commissioning of the field devices.
Known applications of bus systems include all plants and manufacturing facilities where analog signals have been used for communicating measured values. This includes a multitude of areas of application, such as the chemicals industry, petrochemicals, foods, manufacturing in general, etc. The size of the systems reaches from the smallest of plants with around 10 sensors and actuators, up to large plants with hundreds of sensors and actuators.
Often with such applications, older field devices are in use, and, to assure reliable process execution, they must periodically be checked and maintained. Frequently, field devices last for years in use. Some field devices are replaced, for safety's sake, by new ones on the basis of preventative maintenance following a preestablished number of hours of operation, in order to avoid device failure. Thus, it is known that the probability of electronic device failure rises with increasing time in service.
Normally, one strives to replace the field devices with devices of the same construction, since these are easiest to integrate into a running application.
Device manufacturers, however, are constantly developing their products, and, consequently, remove older models from the market, following a certain amount of time.
Thus, it is possible that a device of the same construction will no longer be available for a planned replacement. The user then has two alternatives, either install a model of a newer generation of such devices, or obtain a device of another manufacturer. As a rule, the user decides for a newer model.
A problem that results when an older field device is replaced by a newer model is that it is more complicated to integrate the newer device into the application, i.e. into the automated process. Thus, many bus systems are such that a new device first has to be introduced to the control system, before it can be accepted for use in operation of the automated facility.
The integration of field devices of the same construction is relatively easy, since only slight, or even no, adjustments have to be made concerning the device or the control.
For the integration of field devices of different construction, the controller must be placed in a hold mode and the configuration of the controller so modified that the new device is correctly recognized by the controller. To this end, at least a portion of the process at a plant must be stopped. This is a very undesirable circumstance for the user. Moreover, the configuration of a controller can only be done by specialists and not by the usual operating personnel.
Measures of this character can bring about considerable costs for the user.
An object of the invention is, therefore, to provide a method for operating a field device for automation technology not exhibiting the above-described disadvantages and which, in particular, enables a simple device replacement during ongoing operation, without it being necessary to place a plant in a state of shutdown.
This object is achieved by the provision of at least one alternative identifier, which identifies a similar type of field device, and reporting, on query, to the superordinated unit when the superordinated unit has failed to accept as valid the identifier reported in prior queries.
Advantageous further developments of the invention are set-forth in the dependent claims.
An essential idea of the invention is to store in the field device, along with its own identifier, also other, alternative identifiers, which identify similar field device types, so that if, in the case of a querying by a superordinated unit, the superordinated unit fails to accept as valid the identifier reported in response to an earlier query, then one of the alterative identifiers is reported.
In this way, during initialization, when the superordinated unit does not accept the reported identifier, then an alternative identifier is reported to the superordinated unit. For the control program in the superordinated unit, there is no difference as compared to the state before device replacement. Thus, for the superordinated unit, it seems as if a device exchange has not transpired. The superordinated unit still “sees”, in effect, the old field device. Consequently, no changes are required in the superordinated unit.
Advantageously, the bus system is a PROFIBUS DP or a PROFIBUS PA.
Normally, the superordinated unit is a controller (e.g. a PLC functioning as a Master Class 1 device).
In an alternative embodiment, the superordinated unit assigns a Profibus identification number (Ident-No.) to the field device functioning as a slave device.
Advantageously, a change of the identifier is only permitted during the initialization phase of the communication of the field device.
The invention will now be explained in greater detail on the basis of an example of an embodiment shown in the drawing.
The method of the invention is especially suited for device replacement in the case of field devices for automation technology.